Cultured mouse fibroblasts which are transformed by RNA viruses, a DNA virus or a chemical agent, all secrete a 39,000 Mr-phosphoglycoprotein (major excreted protein, MEP) in large amounts. Nontransformed murine fibroblasts secrete MEP after treatment with tumor promoters such as TPA or growth fators such as PDGF. Secreted MEP appears to be the precursor form of an acid protease with broad specificity. We have purified MEP, prepared monospecific affinity-purified antisera against it and cloned a cDNA which codes for MEP from Chinese hamster and mouse cells. The protein contains mannose 6-phosphate, thelysosomal recognition marker. It is processed intracellularly in both transformed and nontransformed cells to give two specific lower molecular weight forms, the lowest of which has a predominantly lysosomal localization. Transformation, TPA and PDGF stimulate MEP synthesis by increasing levels of MEP specific mRNA. The mechanism of the increase in MEP mRNA levels is increased transcription as measured in nuclear run-off experiments. We are studying this system as a model of regulation of lysosomal protease synthesis, processing and secretion as it is affected by transformation and agents which mimic the transformed state, such as tumor promoters and growth factors.